The 3 dB, 90 degree, hybrid coupler is a 4-port device used for dividing a radiofrequency (RF) signal into two equal amplitude signals with a constant 90 degree phase difference. This style of coupler has two transmission lines passing close enough to each other for energy propagating on one line to couple onto the other line. 3 dB couplers divide the power equally between the two output ports.
There are 4-ports associated with this type of coupler. An input port, a coupled output port, a thru-line output port and an isolation port. Each port is physically the end of the two transmission lines used to create the coupler. The 90 degree phase difference between the two outputs make the couplers useful in a wide array of RF equipment such as variable attenuators, RF mixers, RF modulators, RF amplifiers and many others.
Typically the characteristic impedances (Zo) of each port are the same. In most cases the Zo is 50 or 75 Ohms. This is because the majority of RF systems and devices operate at these two impedances. However there are situations where the input Zo is different that the output Zo, for example, the input may be 25 Ohms and the output may be 50 Ohms. In this case the typical coupler, having Zo equal to 50 Ohms will not work efficiently. Consequently, an impedance transformation must be performed on the 25 Ohm input to raise the impedance up to 50 Ohms before using a typical Zo=50 Ohm coupler.
Conventionally, this impedance transformation requires additional components. However, each connection point, either between a signal line and an RF component, or between multiple RF components, can lead to signal degradation resulting from signal leakage and increased noise from signal reflection at the connection point. Consequently, RF engineers strive to limit the number of mechanical connections in an RF system.